There are various procedures known in the prior art for the preparation of refractory composite structures which are resistant to degradation through oxidation or applied thermal and mechanical stresses or under severe temperature conditions. Such refractory structures can incorporate the use of metal powders such as those used in powder metallurgy processes, ceramic powders and mixtures of ceramic and metal powders commonly referred to as ceramals and cermets. Such products are employed in high temperature environments up to 3000.degree. F. and even beyond as components in turbine engines and heat exchangers. They are also used in low temperature structures requiring characteristics such as high strength/weight ratios, high corrosion resistance, high erosion resistance, and high dielectric capacities. Such materials find uses in the electronics industry and in various bearing applications.
Procedures using in forming high performance structures include isostatic pressing, uniaxial pressing, injection molding, and slip casting procedures. The isostatic and uniaxial pressing procedures may be carried out as "hot" or "cold" procedures. In the former, the pressing operation is carried out at high temperatures, in some cases under sintering conditions, requiring the use of extremely high pressure, high temperature autoclave equipment. In most shaping operations, the composite components are shaped and pressed in a dry powder form. In slip casting, however, a dispersion of the particulate components, sometimes but not always in the colloidal size range, is formed in a thickened liquid suspension, termed a "slip". The slip is then incorporated into a plaster of paris mold and the liquid in the suspension is extracted from the slip by capillary absorption into the interstitial pore spaces of the mold. Pressure assisted slip casting may be employed in which additional pressure is imposed upon the slip within the mold to force the fluid medium into the surrounding mold structure.
Various materials may be employed in producing ceramic composite structures. A conventional approach is to incorporate reinforcing filaments into a particulate matrix material of matrix powders which may include one or more ceramic materials. For example, U.S. Pat. No. 4,543,345 to Wei discloses a refractory composite and its method of preparation in which monocrystalline silicon carbide whiskers are used to reinforce the composite material based upon ceramic matrix powders such as Al.sub.2 O.sub.3, 3Al.sub.2 O.sub.3.2SiO.sub.2, and B.sub.4 C. The silicon carbide whiskers are characterized as having an average diameter of 0.6 microns, a length of 10-80 microns, and an average aspect ratio (the ratio of whisker length to whisker diameter) of 75.
Wei discloses two general procedures for forming the composite. The first, to produce a product in which the whisker orientation is in a plane orthagonal to a pressing axis, is exemplified by the procedure in which fine ceramic powders (0.5-1.0 micron) and silicon carbide whiskers are mixed in hexane and then agitated in a blender followed by dispersion in an ultrasonic homogenizer. The resulting mixture is dried and then hot pressed to a density of more than 99% of theoretical density. Hot pressing is carried out at temperatures of 1600 to 1950.degree. C. and pressures of 28-70 MPa. An alternative to the use of hexane as a solvent in this procedure is distilled water which is removed by freeze drying prior to the hot pressing step. An alternative procedure designed to achieve omnidirectional whisker orientation involves isostatic hot pressing. Here the pressures and temperatures applied to the mixture in a tantalum can in a high temperature inert-gas autoclave are in the same ranges as those employed in the uniaxial pressing procedure.
U.S. Pat. No. 4,560,668 to Hunold et al discloses the production of shaped composites based upon mixtures of polycrystalline silicon nitride and polycrystalline silicon carbide powders having particle sizes up to 10 microns. The particulate mixture is mixed with a temporary binder and dispersed in a solution of a solvent such as acetone or a C.sub.1 -C.sub.6 aliphatic alcohol and then shaped by known techniques such as die pressing, isostatic pressing, injection molding, extrusion molding or slip casting. After the shaping procedure, which is carried out at room temperature or above, the shaped green composite is heated to a temperature from 300 to 1200.degree. C. prior to an encapsulated isostatic hot pressing procedure. The thermotreatment is employed in order to ensure that gaseous decomposition products from the binders do not interfere or damage the casing employed in the hot isostatic pressing process. The composite materials, enclosed within a suitable casing such as tungsten, glass, etc., are heated in a high pressure autoclave at temperatures within the range of 1800-2200.degree. C. at pressures of from 100 to 400 MPa.
Isostatic compression has long been used in the manufacture of shaped ceramic structures. For example, U.S. Pat. No. 3,577,635 to Bergman discloses an isostatic compression process in which a powder body, e.g., a spiral heating element billet, is disposed in an inner container filled with a pressure medium such as glycerin which in turn is placed within an outer pressure chamber filled with a hydraulic oil. The bottom of the inner, glycerin-filled container is provided with an elastomeric flexible membrane which encloses a compression space at least as large as the decrease in total space taking place within the inner chamber. Alternatively, the bottom of the inner container may be provided with a movable cylinder. In either case the pressure is increased to a suitable value, for example 6000 atmospheres, in order to isostaticly compress the object within the inner container.
U.S. Pat. No. 4,612,163 to Nishio et al discloses a cold isostatic pressing process characterized as being of the "wet bag" type in which an elastic bag is placed in the cavity of a permeable mold support. The mold support is placed within a container which is evacuated in order to produce a vacuum and cause the elastic bag to conform to the walls of the mold cavity. The bag is then filled with suitable composite particulates and placed within a cold isostatic pressing unit where pressures of from 2000-4000 atmospheres are imposed. The resulting molding then may be subject to sintering.
U.S. Pat. No. 4,596,781 to Carpenter disclose a procedure for producing a silicon nitride, ternary oxide composite by techniques which can include cold pressing, isostatic pressing, extrusion, injection molding or slip casting. In an exemplary process disclosed in Carpenter, a ternary oxide composition of hafnia, titania, and zirconia is dispersed in water and mixed in a colloidal state and then formed into a disk shape by press filtering. The resulting composition is dried and crushed. An aqueous dispersion of the crushed particles are treated by sedimentation to recover particles of 1 micron or less. These particles are mixed with less than 1 micron size silicon nitride powder and suspended in an aqueous slurry along with alumina sintering aid and then press filtered to form a disk shaped sample. The resulting powder mixture is dried and sintered in air or nitrogen at 1700.degree. C. to produce a silicon nitride composite of about 98% theoretical density.